At present, standardization of LTE-Advanced (LTE-A) is proceeding in 3GPP (3rd Generation Partnership Project).
In the LTE-A, in order to implement a peak data rate exceeding 1 Gbps, a carrier aggregation technique for carrying out wideband transmission using a plurality of component carriers is adopted.
As illustrated in FIG. 1(A), in the carrier aggregation, a transmitter divides a signal (data) for a receiver into a plurality of signals and carries out an up convert process for the signals obtained by the division (hereinafter referred to sometime as divided signals).
Next, as illustrated in FIG. 1(B), the transmitter allocates the divided signals to a plurality of component carriers CC1 and CC2 having, for example, center frequencies f1 and f2 (f1>f2>0), respectively, to produce a wireless signal and transmits the produced wireless signal to the receiver.
On the other hand, as illustrated in FIG. 1(C), the receiver receives the wireless signal from the transmitter and carries out a down convert process for the divided signals allocated to the component carriers CC1 and CC2.
Then, as illustrated in FIG. 1(D), the receiver synthesizes the divided signals in the base band to restore and reproduce the signal.
As described above, by using the carrier aggregation, it is possible to bind the component carriers to expand the band.
Incidentally, as illustrated in FIG. 2A, the receiver carries out an analog/digital (A/D) conversion process, for example, in sampling time of 1/Δf for the signal of a time waveform g(t) received from the transmitter to reproduce the signal. It is to be noted that Δf represents a sampling frequency in the A/D conversion process.
Here, when the process just described is observed on a frequency axis, as illustrated in FIG. 2B, a spectrum G(f) that is a result of Fourier transform of the time waveform g(t) is converted into a signal G′(f) superimposed for each band of Δf. It is to be noted that the bandwidth of the spectrum G(f) in the example of FIG. 2(B) is 2Δf.
Accordingly, as illustrated in FIG. 3, where the receiver carries out sampling of the signals G1(f1) and G2(f2) allocated to the component carriers CC1 and CC2 with a single sampling frequency, overlapping (refer to a black portion of FIG. 3) in spectrum sometimes appears.
In such a case as just described, the receiver cannot separate the overlapping portion of the spectrum and cannot correctly reproduce an original signal.